1. Field of the Invention
This invention relates to position transducers and rotary motion encoders and methods for encoding machine control information for equipment employing a workpiece transport system. The invention has particular utility for pulse train encoders for providing positional output control signals for equipment such as ink jet printers.
2. Description of Related Art
With the development of equipment incorporating multiple electrically controlled elements, for example color ink jet printers, it has become necessary to supply ever increasing amounts of information to control such elements. For example, in a typical ink jet printer, it is necessary to provide on the order of 15,000 to 30,000 control pulses for each inch of travel of the printing medium. Ink jet printing systems, are very sensitive to displacement error of the transport system. Most systems operate by sensing displacement and firing ink jets when the correct position is reached. This position is usually sensed with an encoder. Errors in the displacement signal from the encoder can create undesirable patterns or loss of resolution in the printing, especially in quarter tone and other sensitive printing tones. Such errors can arise from limitations in encoder resolution and eccentricities in the bearings and shafts of the encoder and the transport system.
Motion encoders are devices which produce an electronic signal whose frequency is proportional to the angular velocity of a member being measured (e.g., a shaft) or which produce control signals to indicate positional information. Conventional encoders employ, for example, a very accurate optical disk. The disk can include a series of slots along its circumference or alternating transparent and opaque segments along its circumference which, when conveyed past a light beam, break the light beam and thereby create a pulse as the optical disc rotates. The frequency of the pulse varies as the speed of rotation of the disk varies or positional information is given as the disk rotates. However, optical disks are expensive to manufacture accurately. The alignment specifications required to achieve desired accuracy increases costs significantly and thus prohibit application in many cases. While the accuracy specification of an optical encoder may be 0.25 minutes of arc, even with extreme care, this accuracy can be achieved in practice only with great care in alignment. The expected accuracy achievable with optical encoders available at acceptable cost is about 1-2 minutes of arc. Thus, such optical encoders are limited with respect to the number of control pulses per revolution which can be recorded on them and, typically, commercially available optical encoders of acceptable size cannot provide more then about 20,000 actual pulses per revolution of the encoder disk. To achieve a greater number of control pulses from optical disks requires electronic enhancement techniques which provide virtual pulses from the actual pulse information recorded on the disk. Such enhanced optical encoders are costly and are likely to introduce positional error.
Inductive-type rotary motion encoders employ an induction principle to create pulses as a rotor is rotated. The principle advantage of inductive type rotary encoders is their tolerance to mechanical alignment. The influence of miscentering and tilt are greatly reduced because the rotor sums the contributions from individual stator coils located around the perimeter thereof. However, inductive type encoders have about the same accuracy and actual pulse number limitations as the previously described optical encoders.
Similarly, widely available magnetic disks, such as those used for personal computers, have been considered but do not provide the amount of position data per revolution of the disk required for equipment such as ink jet printers. To obtain the desired number of control pulses requires a step-up drive to rotate the disk at a multiple of the transport drum or encoder roller rotation. Such step-up systems introduce inaccuracies and this compromise the control resolution available. The use of larger disks to increase the number of control pulses per revolution is undesirable, as such disks (either of the optical or the magnetic type) would be non-standard size (and therefore expensive) and would introduce problems stemming from the inertia of the larger disk. Moreover, magnetic encoding can, over time, become compromised by the effects of static discharge and power interruptions to the equipment.
A further disadvantage of the above-described systems is that the control disks are encoded in separate recording equipment. When placed in service, irregularities resulting from mechanical anomalies in the transport systems driving the encoder can result in timing faults to the controlled element, for example an ink jet printing head. The faults can result in reduction in the quality of the printed image and in recurring, undesirable patterns in the printing.